Added MD5 IA and it's tests.

**What does your instant answer do?**
It returns the MD5 digest of a string.

**What problem does your instant answer solve (Why is it better than organic links)?**
No link will give you the md5 directly. You'd have to write or paste the string AGAIN.

**What is the data source for your instant answer? (Provide a link if possible)**
No data source.

**What are some example queries that trigger this instant answer?**
"md5sum Tell me the md5 of this string"

**Which communities will this instant answer be especially useful for? (gamers, book lovers, etc)**
Anybody.

**Is this instant answer connected to a DuckDuckHack [instant answer idea](https://duck.co/ideas)?**
I don't know, but it's connected to github issue #478

**Which existing instant answers will this one supercede/overlap with?**
None.

**Are you having any problems? Do you need our help with anything?**
I'm not sure I should add myself to the attributions as it was mostly copied from #478

## Checklist
```
[X] Added metadata and attribution information
[X] Wrote test file and added to t/ directory
[X] Verified that instant answer adheres to design guidelines(https://github.com/duckduckgo/duckduckgo-documentation/blob/master/duckduckhack/styleguides/design_styleguide.md)
[] Tested cross-browser compatability
```
Looks like the latest is 2.53? https://duckduckgo.com/?q=Digest%3A%3AMD5+metacpan